In a combined cycle power generation plant a heat recovery steam generator (HRSG) may be used to recover heat exhausted by a separate process such as the operation of a gas turbine engine. The HRSG receives the exhausted gas and uses various heat exchanging components to transfer the heat from the exhausted gas to a working fluid. In certain operations the exhaust gas may contain corrosive elements that may cause damage to the heat exchanging components if the flue gas is cooled below a threshold level. For example, gas turbine operations using high sulfur fuels generate flue gas having a relatively high concentration of sulfur oxides, including sulfur dioxide and sulfur trioxide. Sulfur trioxide forms when sulfur dioxide is oxidized. Gaseous sulfuric acid is then formed when sulfur trioxide combines with water vapor. If cooled below a sulfuric acid dew point, the sulfuric acid gas will form liquid sulfuric acid on HRSG interior surfaces, including heat exchanging element external surfaces and the liquid sulfuric acid will damage the interior surfaces, in particular the heat exchanging element external surfaces. When entering the HRSG the flue gas is at a temperature above the sulfuric acid dew point, and hence the formation of liquid sulfuric acid is not a problem at this location. As the flue gas traverses the HRSG and heat is drawn from the flue gas the temperature of the flue gas cools. In addition to corrosives, water vapor may condense and form liquid water on the heat exchanging elements if the flow of flue gas is cooled below the water vapor temperature. This liquid water may interfere with the heat exchanging process and accelerate the flow process in an undesired manner.
Under conventional HRSG operations, care is taken to prevent the temperature of the flue gas from dropping below the sulfuric acid dew point and/or a water dew point at any location in the HRSG. This can be done by, for example heating the working fluid entering heat exchanging elements disposed within the flow of flue gas such that external surfaces of the heat exchanging elements remain sufficiently warm to prevent the unwanted condensation. However, under thermodynamically optimal operation of a HRSG, the working fluid entering at least one of the heat exchanging elements within the HRSG would be at a temperature below the sulfuric acid dew point and/or the water dew point of the flue gas. In this thermodynamically optimal scenario, the relatively cool working fluid would cause the external surface of the heat exchanging element to be below the dew point until heated. When the flue gas encounters the relatively cool surface, or a local volume within the flue duct that has been cooled by the relatively cool surface, the flue gas cools to below the sulfuric acid dew point. Liquid sulfuric acid then forms on the relatively cool surface of the heat exchanging element. The liquid sulfuric acid then acts as a thermal insulator which mitigates heat transfer from the flue gas to the working fluid. This results in the relatively cool working fluid staying cooler longer, which, in turn, expands the size of the relatively cool surface of the heat exchanging element upon which sulfuric acid will form. Over time this liquid sulfuric acid can damage and/or destroy the heat exchanging element.
One conventional solution to this problem has been to preheat the working fluid entering the heat exchanging element to a temperature above the sulfuric acid dew point. In this case, since the working fluid is already above the sulfuric acid dew point when entering the heat exchanging element, liquid sulfuric acid will not form on the heat exchanging elements. However, heating the working fluid necessarily reduces the amount of heat that can be transferred from the flue gas to the working fluid. This reduction in heat transfer reduces a thermal efficiency of the heat recovery steam generator. Consequently, there is room for improvement in the art.